This invention relates to a power steering control apparatus used for an automobile etc., and in particular to a power steering control apparatus having an improved failsafe electrical system for improving the operating safety thereof during a high speed running upon the occurrence of an abnormal condition.
Generally, it is required that a power steering control apparatus for an automobile reduce (lighten) the manual steering power required in proportion to the steering load while increasing the manual steering power required in proportion to the car speed so as to make the steering operation safe during a high speed running.
Such a power steering control apparatus is generally illustrated in FIG. 1 wherein a pump 1 supplies a steering oil pressure Pc through a power steering control valve 2 to a power cylinder 3. The steering oil pressure Pc is also supplied to a reaction chamber 4, which is disposed in the power steering control valve 2, through a hydraulic (oil pressure) control valve 5. The oil pressure Pc is controlled by the operation of the valve 5 actuated by a proportional solenoid 6 which receives as inputs, through an electrical control circuit 7, pulses corresponding to a car speed signal from a car speed sensor 8. The output of the hydraulic control valve 5, namely the controlled oil pressure Pr, is supplied to the reaction chamber 4 to develop a manual steering power in a steering wheel according to a steering load and a car speed. One example of the details of the electrical control circuit 7 is described in the prior U.S. patent application Ser. No. 664,416, titled "Power Steering Control Apparatus" by the same inventors as this invention, filed on Oct. 24, 1984, and is incorporated herein for reference.
In the event that one end of the coil of the proportional solenoid or an electric wire in the car is grounded due to the failure of the wire from the electrical control circuit 7 to the proportional solenoid 6 or of the proportional solenoid 6 per se, the electrical control circuit 7 as above noted includes a failsafe circuit operative to an abnormal feedback condition in which the output voltage of an error amplifying circuit which compares a voltage in inverse proportion to a car speed with a feedback signal from the proportional solenoid is detected whereby an electrical current to the proportional solenoid 6 is interrupted by means of a protection relay unless the output voltage of the error amplifying circuit decreases to zero in a predetermined time interval and the interrupted state is held until a key switch is turned off in order that the proportional solenoid 6 is not burned due to an excessive current flowing through the coil 6 caused by the above ground fault.
The coil resistance of the proportional solenoid 6 is selected so that approximate by a current double the rated current may flow through the solenoid coil when a DC power source (battery) of, for example, 13 V is applied thereto a failsafe circuit is not activated even though the coil resistance increases by approximately a factor of 0.5 due to the thermal increase owing to an excessive current flowing through the coil.
However, the battery voltage may be reduced to i.e., 6 V on such an occasion as operating a starter motor to start the engine and/or the current through the coil of the proportional solenoid 6 may decrease due to the solenoid being heated. In this state, a predetermined current necessary for the definite operation of the solenoid 6 might not flow through the solenoid. Therefore, even when a predetermined time interval lapses in this state, the output voltage of the error amplifying circuit does not decrease so that the failsafe circuit is erroneously activated to interrupt the power source for the proportional solenoid 6 until the key switch is turned off, disadvantageously resulting in a heavy steering load irrespective of the car speed.
In this connection, Japanese Patent Application Laid-open Nos. 55-94854 and 55-39892 disclose a control system which electrically controls a main operating cylinder for a power steering.